Testing an Ap-like Magnetic Braking Origin for the Extended Main Sequence in Young Open Clusters

Abstract

Young open clusters commonly exhibit extended or split upper main sequences, which are widely interpreted as signatures of broad, and in some cases bimodal, stellar rotation distributions. The physical origin of the component commonly associated with slow rotation in this framework remains debated. One proposed channel invokes merger-induced magnetic braking analogous to that seen in Ap stars, which predicts that a large fraction of the relevant stars should host strong, stable surface magnetic fields and exhibit the Ap-like chemical peculiarity responsible for the characteristic $5200\,\mathrmÅ$ flux depression. We test this prediction using Gaia XP spectra of A-type stars in a sample of eight young open clusters and a diagnostic of the $5200\,\mathrmÅ$ flux depression. If Ap-like magnetic braking made a dominant contribution to the extended main-sequence phenomenon, a substantially enhanced incidence of $5200\,\mathrmÅ$ depression would be expected. Instead, we find no evidence for such a large excess. These results disfavour Ap-like magnetic braking as the dominant explanation for the extended main sequence in young open clusters, while leaving open magnetic or non-magnetic channels that would not produce a clear Ap-like $5200\,\mathrmÅ$ signature.